1. Field of the Invention
This invention relates to an optical arrester capable of bypassing an incident light with an electric field intensity higher than an allowable limit, e.g., in optical communication, to protect a subsequent optical device, apparatus or system; particularly a waveguide type of optical arrester automatically controlling an optical output via a waveguide formed on an upper surface of a substrate.
2. Description of the Related Art
Optical communication using light as a means for communication has been being developed for a practical use; advanced multi-functional optical communication systems with large capacity have been being developed. In particular, a wavelength-division multiplex communication (hereinafter, referred to as a WDM communication) has been paid more attention, which transmits information via multiple wavelengths utilizing properties of light. Coming of an optical amplifier into practical use has realized a loss-less transmission path, resulting in rapid increase in a transmission distance. In a long-distance optical communication system in which optical amplifiers are connected in a form of multistage, wavelength-dependency of a gain of an optical amplifier may, however, cause a level difference between channels with different wavelengths. Thus, accumulation of gain in an optical amplifier connected in a form of multistage may resultantly lead to excessive amplification of an electric field intensity of the light, which is larger than a maximum allowable limit of subsequent systems and optical devices. To date there are, however, no effective protectors for protecting a system or an optical device as a part of a system against an electric field intensity (power) of a transmitted incident light in an optical transmission such as an optical communication and an optical interface.
Conventional measures usually conduct a feed-back control; for example, attenuating an optical intensity or switching an optical path with an optical switch when necessary, monitoring an output light from an optical amplifier. For example, an output level of an LD light or a gain level of an optical amplifier may be subject to feed-back control with monitoring an output light from the amplifier. Another example is controlling a transmitted output level of a variable optical attenuator subsequent to an optical amplifier in accordance with monitoring data obtained with keeping a gain of the optical amplifier constant. Both of these are system configurations suggested for fixing, i.e. making equivalent, an electric field intensity of a signal light (an incident light) conducted in a transmission path whose wavelength is multiplexed, between channels. Such configurations may be also utilized as a protector (a security system).
For example, in the General Meeting of Electronic Information Communication Association, 1996, a presentation titled "Investigation of an EDFA Gain Equalizing Circuit utilizing an Array Lattice Type of Filter (B-1183)", a method as shown in FIG. 6 has been presented, in which a variable optical attenuator for each wavelength channel is placed between a preamplifier 122 and a postamplifier 120 in an optical repeater. The gain equalizing circuit shown in FIG. 6 controls optical attenuators 102, 104, 106 and 108 via a counter 118 in accordance with an intensity of each wavelength detected by monitors 110, 112, 114 and 116.
There have been suggested a wide variety of optical device structures such as the above variable optical attenuator, including from those utilizing an optical technique such as lenses and mirrors to an optical waveguide device utilizing an integrated optical technique. The optical waveguide device is of single mode waveguiding. Therefore, it has an advantage that it can easily control with electro-optic or acoustooptic effects, making it applicable to a variety of applications such as optical communication and optical interconnection.
FIG. 7 is a perspective view of an example of an optical waveguide type of optical device, a directional coupler type of optical switch suitable to, for example, switching an optical path. Optical waveguides 72 and 73 are formed by thermal diffusion of metal such as Ti on an LiNbO.sub.3 crystal substrate which is cut vertically to an optical axis and shaped. These optical waveguides 72 and 73 are closely placed in a distance of several micrometers, forming an optical directional coupler 75. On the optical waveguides 72 and 73, a control electrode 76 is formed via a buffer layer (not shown). Here, the optical waveguide always has a constant refractive index to a transmitted radiation. A fundamental principle of operation will be described. First, an incident light 79 enters from an end face of one optical waveguide, e.g., the optical waveguide 73. The light is transmitted through the optical waveguide 73, during which there occurs energy transfer to an adjacent optical waveguide 72 in the area of the optical directional coupler 75. When the length L of the coupling area of the optical directional coupler 75 is equal to a complete connecting length Lc, almost 100% of the energy is transferred to the optical waveguide 72, giving an outgoing light.81. When voltage is applied to the control electrode 76, the refractive indices of the optical waveguides 72 and 73 are changed and become asymmetric each other due to an electro-optic effect, which causes phase mismatching between the incident lights transmitted through both waveguides, leading to change in the coupling state, and thus by applying an appropriate voltage energy is transferred to the original optical waveguide 73 to generate an outgoing light 82. Thus, applying voltage may cause switching of an optical path. Such an optical waveguide device is generally connected with two optical fibers at either of an inlet and an outlet to be used as an optical switch for switching 2.times.2 optical paths. When optical fibers are connected with the optical waveguide 73 as an inlet and the optical waveguide 72 as an outlet, the device may be used as a variable attenuator.
We have described the prior art of the technique for protecting an optical device or system from a light wave with an intensity higher than a maximum allowable limit. In contrast, this invention, as described later, employs an optical waveguide comprising non-linear medium whose refractive index is variable depending on an electric field intensity of an incidence, which can automatically and effectively bypass a signal light with electric field intensity higher than an allowable limit, from a transmission path without any external control.
The above technique of the prior art, however, has problems as follows. First, a monitor is required. Specifically, a light source or an optical amplifier should be feedback controlled, for detecting the electric field intensity of the transmitted signal light based on information from the monitor to switch the light to a bypass or attenuate its electric field intensity, leading to a more complex apparatus (system) and increase of the number of components. Secondly, when highly precise control is required, circuits for detection and control are necessary in accordance with the number of the channels, making the control system more complex and unsuitable for commercial use. As described above, any of the conventional control procedures has the problem that it makes the system more complex, to cause reduction in reliability, productivity and also maintainability.
An optical device employing a non-linear medium has been disclosed in JP-A 03-174523, which is a technique related to an optical neuro device. The device comprises an input optical waveguide and an output waveguide as well as a parallel plate placed between the waveguides which is made of a non-linear medium. The device has the following functions; once an intensity of an input light becomes higher than a certain level, the refractive index of the parallel plate is changed to be in a resonance state, when a strong coupling is generated between the input and the output optical waveguides to substantially increase the intensity of the output light. In other words, it has a function that it causes a resonance state in the parallel plate made of the non-linear medium to increase the intensity of the output light. In this device, the non-linear medium is used as a member for generating the resonance state, rather than as an optical waveguide. On the other hand, the device of this invention employs a non-linear medium as an optical waveguide and has an advantage that once an intensity of an input light reaches a certain level, the light is automatically guided to a bypass. Thus, the device disclosed in the above application is different from that of this invention in its objects, construction and effects. Furthermore, JP-A 62-502782 has disclosed a technique related to a device comprising an optical waveguide with low loss. In this device, an excessive transmission loss is reduced by selectively placing a non-linear medium at a site where excessive transmission loss (e.g., radiation loss, mode conversion loss) may occur, such as a curve in an optical waveguide with a constant refractive index or a site changing the width of the waveguide. Furthermore, the device is not one in which the waveguide itself is made of a non-linear medium. Therefore, it is different from that of this invention in its objects, construction and effects.